1. Field of the Invention
The present invention relates to an inspection method for a projection optical system, which requires highly accurate image formation characteristics, for fabricating semiconductor integrated circuits or liquid crystal devices, and also relates to an inspection apparatus for carrying out the inspection method and a projection exposure system provided with the inspection apparatus. The invention is applicable to a stepper type projection exposure apparatus but is particularly suitable for a projection exposure apparatus of the scan exposure type, such as a slit-scan type or step-and-scan type, where patterns of geometric shapes on a mask are serially transferred on a photosensitive substrate while scanning in synchronization the mask and the substrate.
2. Description of the Related Art
Projection optical systems, which are mounted, for example, in a projection exposure apparatus for fabricating semiconductor integrated circuits or liquid crystal devices, require extremely high accuracy with respect to image formation characteristics such as projection magnification and image distortion. For this reason, a method for measuring projection magnification and image distortion of the projection optical system with high accuracy and a correction method for correcting the image formation characteristics with high accuracy have been developed. At present, there are roughly two methods for measuring projection magnification and image distortion.
The first method is one which transfers a pattern of a test mask to a photosensitive substrate (e.g., wafer). This method is disclosed, for example, in Japanese Patent Laid-Open Publication No. Sho 58-8353. In the first method, the test mask pattern is transferred onto a photosensitive substrate, and after the photosensitive substrate is moved a predetermined distance in accordance with a laser interferometer, the pattern is again transferred onto the substrate so that it overlaps with the previously transferred pattern. After development of the substrate, the overlap error or registration error is measured. At this time, since marks overlap one above the other after the photosensitive substrate is moved, the marks are patterns drawn at different places on the test mask.
The second method is one where an image of a pattern formed on a photosensitive substrate is measured directly by a photoelectric sensor without an actual exposure process such as the first method. This second method is disclosed, for example, in Japanese Patent Laid-Open Publication Nos. Sho 59-94032 or Sho 60-18738. An example of the second method will be briefly described in reference to FIGS. 1(a) and 1(b).
FIG. 1(a) shows a schematic structure of an example of a conventional projection exposure apparatus. As shown in FIG. 1(a), a test reticle TR as a test mask is provided with a plurality of light transmission portions 305A to 305B (in this example, slits) formed at predetermined intervals. Illumination light passes through the light transmission portions 305A to 305G and forms an image thereof on the photosensitive substrate side through a projection optical system PL. FIG. 1(a) shows the arrangement of a pattern plate 301 and a photoelectric sensor 302 (both of which are positioned on an image forming position of the light transmission portion 305A). The pattern plate 301 has a very small light transmission portion 306 which serves as a mark detection device. The photoelectric sensor 302 receives the illumination light from the light transmission portion 306. The pattern plate 301 and the photoelectric sensor 302 are mounted on a wafer stage, which has the photosensitive substrate mounted thereon and is movable on a plane perpendicular to an optical axis of the projection optical system PL. The position of the wafer stage is precisely measured by a reflector 303 fixed to the wafer stage and an external laser interferometer 304. The output of the photoelectric sensor 302 varies by scanning the wafer stage.
FIG. 1(b) shows a graph representing the result of the output of the photoelectric sensor 302. In the figure, the axis of abscissa represents a position x of the scanning direction of the wafer stage, and the axis of ordinate represents an output value I of the photoelectric sensor 302. The image forming position of the light transmission portion 305A of the test reticle TR can be measured by obtaining, in FIG. 1(b), a position x0 where the output value I of an output curve 307 becomes maximum. If a similar measurement is performed with respect to a plurality of light transmission portions of the test reticle TR, the respective image forming positions of the light transmission portions 305A to 305G will be obtained. Therefore, the projection magnification and the image distortion of the projection optical system can be obtained because the positions of the light transmission portions 305A to 305G are known in advance.
In addition to the aforementioned photoelectric sensor scanning method, there is known a method where an image of a pattern on a reticle is magnified with a microscope and is detected by mean of an image pick-up device, such as two-dimensional CCD, or a method where, conversely, illumination light is emitted from a slit provided on a wafer stage and is received via a pattern of a test reticle TR by scanning the wafer stage (see Japanese Patent Laid-Open Publication No. Sho 63-81818).
The image formation characteristic of the projection optical system, such as magnification or image distortion, is required to be measured and regulated at the time of the manufacture by a projection exposure apparatus. The image formation characteristic also is required to be corrected at the time of actual use, because it varies due to an atmospheric pressure variation and illumination light absorption of a projection optical system. As a countermeasure, a method, where a quantity of the variation of the image formation characteristics are predicted in advance and correction of magnification is performed by varying an air pressure of the projection optical system, is known as disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 60-28613 or Sho 60-78457.
However, this method alone is insufficient and also there is the possibility that magnification and image distortion vary due to long-term fluctuations in a system. Therefore, the system should be used while periodically checking magnification and image distortion by means of methods such as described above. Also, since the demand for accuracy of correction corresponding to an atmospheric pressure variation has become increasingly severe in recent years, it is necessary to frequently check a correction error caused by measurement. In this sense, the aforementioned second method whose measurement time is short is superior to the first method, and in many cases, the second method is actually used.
Also, a step-and-scan projection exposure apparatus, where a mask and each shot area of a photosensitive substrate are scanned in synchronization with respect to a projection optical system in order to substantially increase an exposure area without greatly increasing the size of the projection optical system, has been aimed at in recent years. However, there has been provided no method for measuring an image formation characteristic, which makes use of, in particular, the feature of a projection exposure apparatus of a scan exposure type such as a step-and-scan type.
Thus, all of the aforementioned conventional methods of measuring the magnification or image distortion of the projection optical system are a method of measuring a gap or distance between the projected images of two (or more) different marks on a test mask. For this reason, it is necessary to accurately grasp the mark gap or pitch of the test mask in advance. However, normally the patterns on the mask are fabricated with an electronic beam drawing device, and the gap or pitch between spaced marks for measurement of magnification is not very accurate, so the gap of each mask has to be measured in advance with a reticle pattern measuring machine. This measurement is substantially impossible in a manufacturing site where a plurality of masks are used. For this reason, it is conceivable to use a reference mask, but this method has the disadvantage that measurement cannot be performed frequently during the aforementioned actual exposure.
Also, the above method has a problem regarding accuracy of reticle pattern measurement. For example, when a mask is mounted in an exposure apparatus, it is normally disposed with the pattern thereof facing downwardly, but in the reticle pattern measuring machine the mask is mounted with the pattern surface thereof facing upwardly. Therefore, influences of deflection caused by self-weight are different between the two masks. This difference results in a measurement error. In addition, even in the exposure apparatus, deflection by self-weight varies between the masks and causes image distortion.
Furthermore, although it is also conceivable that only some of the masks are measured in order to perform correction of the projection optical system, the gap or distance between the marks varies because the mask absorbs the heat of the illumination light during measurement and therefore the mask itself is thermally expanded. If correction, including the thermal expansion of the mask, is made during measurement, there will be no problem. However, if the mask is exchanged for a subsequent mask, there will be the drawback that an error remains in the magnification of the projection optical system, because the mask after exchange has not been thermally expanded. Moreover, even in a method where the distance between the marks is unknown but magnification is always held in an initial magnification obtained at the time of exchange, if a mask is exchanged for a subsequent one, an error will occur in the magnification of the projection optical system, for the same reasons.
A primary object of the present invention is to provide an inspection method which is capable of accurately measuring an image formation characteristic of a projection optical system, such as magnification and image distortion, while overcoming drawbacks of conventional inspection methods such as described above.
Another object of the present invention is to provide an inspection method which could be capable of accurately measuring an image formation characteristic of a projection optical system, such as magnification and image distortion, even if there are drawing errors in marks for projection magnification and image distortion measurement formed on a mask.
Still another object of the present invention is to provide an improved inspection apparatus which is capable of accurately and quickly measuring an image formation characteristic of a projection optical system, such as magnification and image distortion.
A further object of the present invention is to provide a projection exposure apparatus having an inspection apparatus which is capable of accurately and quickly inspecting an image formation characteristic of a projection optical system, such as magnification and image distortion.
A further object of the present invention is to provide a projection exposure apparatus which is capable of accurately and quickly measuring an image formation characteristic of a projection optical system, such as magnification and image distortion, and also accurately correcting the image formation characteristic.
A first inspection method for a projection optical system according to the present invention comprises the steps of (1) moving a predetermined pattern formed on a mask to a first position, (2) moving the predetermined pattern to a second position different from the first position perpendicularly with respect to an optical axis of the projection optical system, (3) detecting a positional relationship between the first position and the second position, (4) detecting a positional relationship between a position where the image of the predetermined pattern in the first position is projected by the projection optical system and a position where the image of the predetermined pattern in the second position is projected by the projection optical system, and (5) obtaining an image formation characteristic of the projection optical system, based on the positional relationship detected in step (3) and the positional relationship detected in step (4).
A second inspection method for a projection optical system according to the present invention comprises the steps of (1) detecting a positional relationship between first and second patterns formed on a mask, (2) detecting a positional relationship between a position where an image of the first pattern is projected by the projection optical system and a position where the image of the second pattern is projected by the projection optical system, and (3) obtaining an image formation characteristic of the projection optical system, based on the positional relationship detected in step (1) and the positional relationship detected in step (2).
In the aforementioned first or second inspection method of a projection optical system according to the present invention, an example of the projection optical system is provided in a projection exposure apparatus which transfers the pattern formed on the mask to a photosensitive substrate.
A third inspection method for a projection optical system according to the present invention comprises the steps of (1) detecting a positional relationship between a plurality of reference marks, the reference marks being defined by a reference mark member disposed on a mask stage and also being arranged at intervals in a moving direction of the mask stage, (2) forming projected images of the reference marks on a wafer stage by illuminating a collimated light beam to the reference marks of the reference mark member, (3) moving the wafer stage and then detecting a positional relationship between a plurality of projected images of the reference marks, and (4) obtaining an image formation characteristic of the projection optical system, based on the positional relationship between reference marks and the positional relationship between projected images detected in step (3).
In the aforementioned inspection method, the reference mark member may be a single reference mark member, and the plurality of reference marks may be formed in the single reference mark member. Also, a center of the single reference mark member may be aligned with an optical axis of the projection optical system. In addition, the reference mark member may comprise a plurality of reference mark members, and the plurality of reference mark members may be provided around a position where the mask is arranged.
A fourth inspection method for a projection optical system according to the present invention comprises the steps of (1) detecting a positional relationship between a plurality of reference marks of a reference mark member provided in positions which are optically conjugate with patterns of a mark arranged on a mask stage, the reference marks being arranged at intervals in a moving direction of the mask stage, (2) forming projected images of the reference marks on a wafer stage by illuminating a collimated light beam to the reference marks of the reference mark member, (3) moving the wafer stage and then detecting a positional relationship between a plurality of projected images of the reference marks, and (4) obtaining an image formation characteristic of the projection optical system, based on the positional relationship between reference marks and the positional relationship between projected images detected in step (3).
A first inspection apparatus for a projection optical system according to the present invention comprises: a mask stage arranged on the incident light side of the projection optical system to be inspected and movable in a direction perpendicular to an optical axis of the projection optical system, while holding a mask; a position measuring device for measuring first and second positions of the mask stage; and an image position detection device for detecting a relationship between a projected position at which an image of a predetermined pattern formed on the mask is projected through the projection optical system when the mask stage is in the first position and a projected position at which an image of the predetermined pattern formed on the mask is projected through the projection optical system when the mask stage is in the second position.
A second inspection apparatus for a projection optical system according to the present invention comprises: a mask stage arranged on the incident light side of the projection optical system to be inspected and holding a mask having first and second patterns; a pattern position detection device for detecting a positional relationship between the first and second patterns; and an image position detection device for detecting a relationship between a projected position at which an image of the first pattern is projected through the projection optical system and a projected position at which an image of the second pattern is projected through the projection optical system.
A third inspection apparatus for a projection optical system according to the present invention comprises: a reference mark member provided on a position on a mask stage or a position which is optically conjugate with patterns held in the mask stage and formed with a plurality of reference marks; an illumination optical system for projecting images of the plurality of reference marks toward the projection optical system to be inspected; and an image position detection device for detecting a position of a projected image of at least one of the plurality of reference marks obtained through the projection optical system.
A fourth inspection apparatus of a projection optical system according to the present invention comprises: a reference mark member having a plurality of reference marks formed thereon and disposed on a mask stage adjacent to a mask in a scanning direction of the mask stage; an illumination optical system for projecting images of the plurality of reference marks toward the projection optical system to be inspected; and an image position detection device for detecting a position of a projected image of at least one of the plurality of reference marks obtained through the projection optical system.
A first projection exposure apparatus according to the present invention comprises: a projection optical system for projecting an image of a pattern formed on a mask on a predetermined plane; a mask stage freely movable in a direction perpendicular to an optical axis of the projection optical system, while holding the mask; a position measuring device for measuring a position of the mask stage; a stage control device for moving the mask stage from a first position to a second position, based on an output of the position measuring device; an image position detection device for detecting a positional relationship between a projected position at which an image of the predetermined pattern formed on the mask is projected through the projection optical system when the mask stage is in the first position and a projected position at which an image of the predetermined pattern formed on the mask is projected through the projection optical system when the mask stage is in the second position; and a calculation device for calculating an image formation characteristic of the projection optical system, based on the positions of the mask stage measured by the position measuring device when the mask stage is in the first and second position and based on the positional relationship detected by the image position detection device.
A second projection exposure apparatus according to the present invention comprises: a mask stage for holding a mask having first and second patterns formed thereon; a projection optical system for projecting images of the patterns on a mask onto a predetermined plane; a pattern position detection device for measuring a positional relationship between the first and second patterns; an image position detection device for detecting a positional relationship between a projected position at which an image of the first pattern is projected through the projection optical system and a projected position of an image at which the second pattern is projected through the projection optical system; and a calculation device for calculating an image formation characteristic of the projection optical system, based on the positional relationship detected by the pattern position detection device and the positional relationship detected by the image position detection device.
In the aforementioned projection exposure apparatuses according to the present invention, an example of the pattern position detection device has a pattern detection device for photoelectrically detecting the first and second patterns of the mask, a stage control device for moving the mask stage perpendicularly relative to an optical axis of the projection optical system so that the first and second patterns cross an area of detection of the pattern detection device, and a position measuring device for measuring a position of the mask stage.
Also, the aforementioned first and second projection exposure apparatuses of the present invention further comprise a substrate stage for mounting a photosensitive substrate to which a predetermined pattern of the mask is transferred. In addition, an example of an object to be detected by the image position detection device is an image of the predetermined pattern or first and second patterns transferred to the photosensitive substrate.
Furthermore, it is preferable that the aforementioned first and second projection exposure apparatuses of the present invention are a projection exposure apparatus of the scan exposure type where a photosensitive substrate is exposed while scanning the photosensitive substrate and a mask in synchronization with each other.
A third projection exposure apparatus according to the present invention is suitable for projecting a pattern on a mask mounted on a mask stage onto a photosensitive substrate through a projection optical system and comprises: a reference mark member formed with a plurality of reference marks and disposed in a position on the mask stage or a position which is nearly optically conjugate with the pattern; an image position detection device for detecting a position of a projected image of at least one of the plurality of reference marks under illumination light in the same wavelength band as illumination light for exposure; a calculation device for obtaining an image formation characteristic of the projection optical system, based on the result of the detection of the image position detection device; and a correction device for correcting an image formation characteristic of the projection optical system, based on the image formation characteristic obtained by the calculation device.
A fourth projection exposure apparatus according to the present invention is suitable for projecting a transfer pattern formed on a mask onto a photosensitive substrate through a projection optical system by scanning the photosensitive substrate in a direction (xe2x88x92X direction or +X direction) corresponding to a predetermined scanning direction (+X direction or xe2x88x92X direction) in synchronization as the mask formed with a pattern to be transferred is scanned in the predetermined scanning direction through the mask stage and comprises: a reference mark member having a plurality of reference marks formed thereon and disposed on a mask stage adjacent to a mask in a scanning direction of the mask stage; an image position detection device for detecting a position of a projected image of at least one of the plurality of reference marks under illumination light in the same wavelength band as illumination light for exposure; a calculation device for obtaining an image formation characteristic of the projection optical system, based on the result of the detection of the image position detection device; and a correction device for correcting an image formation characteristic of the projection optical system, based on the image formation characteristic obtained by the calculation device.
In the aforementioned projection exposure apparatus according to the present invention, it is preferable that the reference mark member be arranged within an approach run section of the mask stage where the illumination light for exposure is illuminated, at the time of acceleration or deceleration during scan and exposure.
Also, in the aforementioned third and fourth projection exposure systems according to the present invention, an example of an object of detection of the image position detection device is an image projected on a test photosensitive substrate or a test thermosensitive substrate through the projection optical system.
In accordance with the aforementioned first inspection method of the present invention, the image formation characteristics of the projection optical system, such as magnification and image distortion, could be accurately detected even if there is a drawing error in the mask on the mask for measuring a projection magnification or an image distortion.
That is, the principles of measuring the projection magnification or the image distortion of the projection optical system is to measure how a given length on the mask side varies through the projection optical system. Therefore, it is necessary that a length which becomes single reference on the mask side is accurately known. In the first inspection method of the present invention, a predetermined pattern on the mask is moved. To obtain the positional relationship (distance of movement), a distance that this pattern is moved becomes a single strict length standard on the mask side. And, to obtain the positional relationship between the images of this predetermined pattern obtained on a predetermined plane through the projection optical system before and after the movement of the predetermined pattern, the relation between a length which becomes a single standard on the mask side and a variation in the length through the projection optical system, i.e., the image formation characteristics of the projection optical system is strictly obtained.
Also, in accordance with the aforementioned second inspection method for the projection optical system of the present invention, a length which becomes a single standard on the mask side is strictly known by obtaining the positional relationship (gap) between the first and second patterns formed on the mask. This method, as with the aforementioned first inspection method of the present invention, could accurately detect the image formation characteristics of the projection optical system, such as magnification and image distortion, even if there is a drawing error in the mark on the mask for measuring projection magnification or an image distortion.
In addition, in the first and second inspection methods for the projection optical system of the present invention, in a case where the projection optical system is provided in a projection exposure apparatus where a pattern formed on a mask is transferred onto a photosensitive substrate, the image formation characteristics of the projection optical system can be accurately detected. If correction is performed based on the detection, the mask pattern can be transferred with a high degree of overlap or registration accuracy to the photosensitive substrate.
Furthermore, according to the first projection exposure apparatus of the present invention, the aforementioned first inspection method for the projection optical system can be carried out. That is, accurate measurement of magnification and image distortion can be performed with respect to not only a test mask where a gap or distance between marks is measured in advance but also all masks that are used in actual exposure.
Moreover, according to the fourth projection exposure apparatus of the present invention, the aforementioned second inspection method for the projection optical system can be carried out. That is, the positional relationship between the first and second patterns formed on the mask is accurately detected as needed by means of the pattern position detection device. Therefore, the second projection exposure apparatus of the present invention, as with the first projection exposure apparatus of the present invention, could accurately detect the image formation characteristics of the projection optical system even if masks were different.
In the aforementioned second projection exposure apparatus of the present invention, the pattern position detection device has a pattern detection device for photoelectrically detecting the patterns on the mask, a stage control device for moving the mask stage perpendicularly relative to an optical axis of the projection optical system so that the first and second patterns cross an area of detection of the pattern detection device, and a position measuring device for measuring a position of the mask stage. In such a case, the mask stage is moved by the stage control device, and the first and second patterns on the mask are detected by the pattern detection device. At this time, the position of the mask stage is detected by the position measuring device. In this way, the positional relationship between the first and second patterns can be detected.
Also, in the third and fourth projection exposure apparatuses of the present invention, when the apparatuses further comprise a substrate stage for mounting a photosensitive substrate to which a pattern of the mask is transferred and also an object to be detected by the image position detection device is an image of the predetermined pattern or first and second patterns transferred to the photosensitive substrate, the image formation characteristics of the projection optical system can be accurately detected in the same state as actual exposure.
In addition, when the third and fourth projection exposure apparatuses of the present invention are a projection exposure apparatus of the scan exposure type where a photosensitive substrate is exposed while scanning the photosensitive substrate and a mask in synchronization with each other, the apparatus do not need to be newly modified because normally there is provided a mask stage and a position measuring device which are the constitutional elements of the present invention.
Furthermore, according to the third projection exposure apparatus, the image formation characteristics of the projection optical system is accurately measured without suffering the influence of a drawing error on the mask and is corrected by the correction device.
In the prior art, the image formation characteristics, such as magnification or image distortion, has been measured with the reference mark for measurement of the mask. On the other hand, in the present invention, the reference mark member has a plurality of reference marks whose positions are accurately measured in advance, and when the image formation characteristics of the projection optical system is measured, the reference mark member is moved, for example, up to a position where the mask is held during exposure, and the image formation characteristic, such as projection magnification or image distortion, is measured with the reference marks of the reference mark member. Therefore, unlike the prior art, the image formation characteristics of the projection optical system can be measured and accurately corrected without suffering the influence of a drawing error of the reference mark for measurement on the mask, a picture position measurement error, or thermal expansion of the mask.
According to the fourth projection exposure apparatus of the present invention, in the scan-exposure type projection exposure apparatus, as with the first projection exposure system, the image formation characteristics of the projection optical system can be measured without suffering the influence of a drawing error on the mask and can be corrected by the correction device. Also, since the reference mark member has been arranged on the mask stage in the scanning direction with respect to the mask, the reference mark member can be quickly arranged in a position where the mask is exposed, by moving the mask stage.
In addition, in the fourth projection exposure apparatus of the present invention, when the reference mark member is arranged within an approach run section of the mask stage where the illumination light for exposure is illuminated, at the time of acceleration or deceleration during scan and exposure, there is no need to provide an additional place for the reference mark member. Since the reference mark member is arranged within the approach run section during scan and exposure, illumination light for exposure can be used as illumination light for illuminating the reference mark. Therefore, a measurement error of the image formation characteristics, which is caused due to a difference between illumination light beams, does not occur.
Furthermore, in the aforementioned third and fourth projection exposure systems of the present invention, when an object to be detected by the image position detection device is an image projected on a test photosensitive substrate or a test thermosensitive substrate through the projection optical system, the image formation characteristics of the projection optical system can be accurately detected in the same state as actual exposure.